Preparation and study of high-thermal conductivity phase-change energy-storage materials based on expanded graphite and pitch through high-temperature sintering

The rapid development of global industry has accelerated the consumption of fossil fuels, leading to increasingly severe energy shortages and a growing demand for renewable energy sources [1,2]. Meanwhile, there is a disparity in the supply and demand of energy across time and space, which can be effectively addressed by energy-storage technologies [3,4]. Floor radiant heating systems [5] utilize phase-change latent heat energy-storage technology [6,7]. Phase-change materials (PCMs) manage heat efficiently by absorbing or releasing heat through solid–solid, solid–gas, liquid–gas, and solid–liquid phase transitions. Organic PCMs [[8], [9], [10], [11]] offer advantages such as no supercooling, good stability, and high crystallization rates [12]. However, low thermal conductivity, flammability, and high cost limit their commercial development and application. In contrast, inorganic PCMs (IPCMs) [[13], [14], [15], [16]] are cost-effective and have high latent heat and excellent flame retardancy. Therefore, IPCM was selected as the phase change material in this study, expected to demonstrate significant economic and safety advantages in future production applications.

By integrating IPCM with carriers, its inherent disadvantages, such as high supercooling temperatures [17,18], poor thermal conductivity [19,20], leakage [[21], [22], [23], [24]], and phase-separation failure [25,26], can be mitigated. Wu et al. [27] found that while melamine foam has good adsorption properties, the composite with reduced graphene oxide had a thermal conductivity of only 0.096 W·m⁻¹ K⁻¹. Jia et al. [28] investigated how to prepare thermally cycle-stable phase change material carriers from recyclable materials. Liu et al. [29] showed that treatment of the molecules with PEG strengthens the hydrogen bonding and results in a stronger overall strength of the composite phase change material. Pitch [2] has high carbon content, low cost, soluble in solvents, mixes effectively with additives and binds the components, and has a highly ordered microstructure after sintering. Focke et al. [30] combined expandable graphite with coal pitch and foamed the mixture at 437 °C to produce graphite foam with a theoretical thermal conductivity of 21 W·m⁻¹ K⁻¹, density of 0.249 g·cm⁻³ and compressive strength of 0.46 MPa. However, the sintered graphite foam is essentially mixed with expanded graphite using the characteristics of intermediate-phase asphalt, which is more costly and a more complex preparation process. Additionally, the sintering process at 2600 °C in an atmospheric environment has a higher cost which contradicts the selection of IPCM aimed at reduce the cost.

The innovation of this study is to use expanded graphite as the main raw material and pitch as the binder, and after sintering to obtain a porous 3D network structure skeleton, as well as a low production cost and excellent overall strength. In addition, coating CPCM with commercial thermally conductive epoxy resin (ER) alleviates the leakage issues of solid – liquid PCMs, expanding PCM application scenarios and practical performance.

In this study, modified sodium acetate trihydrate (SAT) was selected as the IPCM [31]. PEGF with high thermal conductivity (2.381 W·m⁻¹ k⁻¹) was prepared by high-temperature sintering method using EG and bitumen as raw materials. Using physical adsorption, MPCM was combined with PEGF to obtain a CPCM with high phase-change latent heat (236.5 J/g), a suitable phase-change temperature (48.86 °C), excellent flexural strength (12.401 MPa) and great compressive strength (22.323 MPa). A high-thermal-conductivity (thermal conductivity >2.9 W·m⁻¹ K⁻¹) ER adhesive was applied to the CPCM to create a PCPCM, which can be used directly as a heat storage body. The microstructure, thermal properties, and mechanical properties of the CPCM were investigated. Cycling tests show that the latent heat and phase-change temperature of the material are basically stable, the phase separation and material segregation have been improved, and the problem of settling of expanded graphite has been solved.